Add integrator/likelihoods doc

[thomaspinder]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

Checklist

• I've formatted the new code by running poetry run pre-commit run --all-files --show-diff-on-failure before committing.
• [-] I've added tests for new code.
• [-] I've added docstrings for the new code.

Description

This PR introduces a piece of documentation that provides users with a guide to likelihood functions and how the integration strategy attached to a likelihood function can be changed.

Issue Number: #295 #294

[daniel-dodd]

To merge:

Looks good @thomaspinder. Left some comments.

For future:

Its nice that you break down the AbstractLikelihood down. As a further suggestion, it would be nice if we explicitly demonstrated what a user would have to code themselves if they wanted to add their own likelihood (like we do in the kernels guide), as an example. E.g., via a Poisson or Bernoulli (or Both). You could even maybe have a table that breaks things down with the maths and the code for the link functions, what the data looks like under each scenario -> e.g., count use Poisson, for Binary use Bernoulli, etc.

And it would be good to explicitly have links/ advertise custom likelihood, kernel, mean function etc, somewhere in our docs.

[thomaspinder]

Thanks @daniel-dodd - all suggestions actioned.